1. Field of the Invention
The invention relates to a novel nanocomposite thermoelectric conversion material, a thermoelectric conversion element including the same, and a method of producing the nanocomposite thermoelectric conversion material. More specifically, the invention relates to a nanocomposite thermoelectric conversion material in which a thermal conductivity is low due to a specific structure of an interface between a matrix of a thermoelectric conversion material and nanoparticles of a dispersed material, a thermoelectric conversion element including the same, and a method of producing the nanocomposite thermoelectric conversion material by liquid phase synthesis.
2. Description of the Related Art
Recently, increased attention has been focused on a technology for reducing the proportion of energy from fossil fuel, in order to reduce the amount of discharged carbon dioxide, taking global warming into account. Examples of the technology include a thermoelectric conversion material that directly converts unused waste heat energy to electric energy, and a thermoelectric conversion element including the same. The thermoelectric conversion material directly converts heat to electric energy, unlike thermal power generation in which heat is converted to kinetic energy, and then, the kinetic energy is converted to electric energy in two steps. The basic structure of the thermoelectric conversion material is described in, for example, Japanese Patent Application Publication No. 11-298052 (JP-A-11-298052).
Heat is converted to electric energy using a difference of temperatures at both ends of a bulk body formed of the thermoelectric conversion material. The phenomenon, in which voltage is generated due to the temperature difference, is referred to as the Seebeck Effect, because this phenomenon was discovered by Seebeck. The performance of the thermoelectric conversion material is represented by a performance index Z determined using the following equation.Z=α2σ/κ(=Pf/κ)
In the equation, α represents the Seebeck coefficient of the thermoelectric conversion material, σ represents the electric conductivity of the thermoelectric conversion material, and κ represents the thermal conductivity of the thermoelectric conversion material. The term α2σ is referred to as an output factor Pf. Z has a dimension that is an inverse of temperature. ZT, obtained by multiplying the performance index Z by an absolute temperature T, is a nondimensional value. ZT is referred to as “nondimensional performance index”. The nondimensional performance index ZT is used as an index indicating the performance of the thermoelectric conversion material. The performance of the thermoelectric conversion material needs to be further improved so that the thermoelectric conversion material is widely used. As evident from the above-described equation, the Seebeck coefficient α needs to be increased, the electric conductivity σ needs to be increased, and the thermal conductivity κ needs to be decreased, to improve the performance of the thermoelectric conversion material.
However, it is difficult to improve all the properties at the same time. Many attempts have been made to improve any one of the above-described properties of the thermoelectric conversion material. For example, Japanese Patent Application Publication No. 2000-261047 (JP-A-2000-261047) describes a thermoelectric conversion material represented by CoSbX (2.7<X<3.4), more specifically, a thermoelectric conversion semiconductor material in which ceramic powder, which is a dispersed material, is dispersed in a matrix of the thermoelectric conversion material that is CoSb3, and a method of producing the thermoelectric conversion semiconductor material, in which source material powder represented by CoSbX is mixed with ceramic powder that is the dispersed material, shape forming is performed, and then, calcinations is performed. However, in the above-described publication, reference is not made to an interface between the matrix of the thermoelectric conversion material and particles of the dispersed material. Also, the thermal conductivity of the thermoelectric conversion material described in the publication is 1.8 to 3 W/Km, although the thermal conductivity is lower than the conductivity (approximately 5 W/Km) of a thermoelectric conversion material that does not contain ceramic powder.
Japanese Patent Application Publication No. 2000-252526 (JP-A-2000-252526) describes a thermoelectric material that is a sintered body including Sb-containing skutterudite compound crystal grains and a metal oxide dispersed in a crystal grain boundary, and a method of producing the same. In the publication, it is described that the thermal conductivity of the thermoelectric material is decreased, and the performance index of the thermoelectric material is improved by miniaturizing the crystal grains. The thermal conductivity of the thermoelectric material described in the publication varies depending on the type of the Sb-containing skutterudite compound crystal grains. However, in any case, the thermal conductivity is equal to or higher than 1.6 W/Km.
Japanese Patent Application Publication No. 2002-26404 (JP-A-2002-26404) describes a method of producing a thermoelectric material, which includes steps of sealing, in a container, the melt of a scattering-center material that scatters phonons, and a matrix; oscillating the container; and cooling the mixture. The publication also describes the thermoelectric material produced using the mixture of the matrix containing at least two elements selected from among Bi, Sb, Co, and the like, and the scattering-center material that scatters the phonons. In the thermoelectric material, the average diameter of the crystal grains is 2 μm to 20 μm, a filling rate is 95 to 100%, and the scattering-center material is uniformly dispersed in the matrix. However, in the publication, reference is not made to an interface between the matrix of the thermoelectric conversion material and particles of the dispersed material.
Published Japanese Translation of PCT application No. 2008-523179 describes a thermoelectric nanocomposite semiconductor material composition that includes a semiconductor host material, and a plurality of nano-sized objects dispersed in the material. A band-edge offset between conduction bands or valence bands at a border between both the materials is smaller than approximately 5 kBT (kB: Boltzmann constant, T: the average temperature of the composition). In the publication, it is described that the thermal conductivity is decreased by mixing the nanoparticles or nanowires into the semiconductor host material, and thus, the performance index is improved; the shape of the nanoparticle is not limited; and the nanowires are irregularly arranged.
Japanese Patent No. 3559962 describes a thermoelectric conversion material in which nanoparticles of the thermoelectric material are dispersed in a solid matrix, and a method of producing the thermoelectric conversion material, which includes the step of irradiating a target material with a laser beam. In the publication, it is described that the thermal conductivity is decreased by replacing the surfaces of nanoparticles of the thermoelectric material by a heterogeneous material (i.e., by modifying the surfaces of the nanoparticles). Also, Japanese Patent No. 3925932 describes a method of producing organically modified metal oxide nanoparticles, in which surfaces of metal oxide nanoparticles are organically modified. Also, in the publication, SiO2 nanoparticles are described. However, in the publication, reference is not made to a thermoelectric conversion material.
In the above-described technologies, the thermal conductivity κ is not sufficiently decreased, and the level of the performance of the produced thermoelectric conversion material is low.